1. Field of the Invention
This invention generally relates to a method and system for monitoring the volumetric output of a heart, and more specifically, to a method and system for making this determination by using a non-invasive technique which monitors the carbon dioxide elimination rate of the patient.
2. Prior Art
Cardiac output, the volumetric rate at which blood is pumped through the heart, is one of the most important cardiovascular parameters. The cardiac output reflects the supply of oxygen and nutrients to the tissues of the patient. Measurements of cardiac output provide invaluable clinical information for quantifying the extent of cardiac dysfunction, indicating the optimal course of therapy, managing patient progress, and establishing check points for rehabilitation in a patient with a damaged or diseased heart, or one in whom fluid status control is essential. Exercise, as well as pathological conditions of the heart and circulatory system will alter cardiac output; therefore, the measurement of cardiac output is useful both in rehabilitation and critically ill patients.
Instrumentation currently in use for measurement of cardiac output has several disadvantages. Cardiac output may be measured either invasively or noninvasively. The most common invasive techniques are indicator dilution and thermal dilution.
The indicator dilution technique typically relies on the use of a dye or other indicator which is injected suddenly upstream of the heart and analyzed downstream of the heart. In the case of a dye, the density of dye in the blood increases initially to a maximum and then decays exponentially. The curve produced establishes an area under the curve indicative of the volume of blood pumped by the heart and the time taken for this blood to pass the point where analysis is being made. Of course before the density of the dye can taper off exponentially to substantially zero, recirculation of the dye will produce a second peak in the density curve introducing a distortion to the decaying exponential portion of the curve. This distortion cannot be tolerated if an accurate reading of cardiac output is to be established. Various methods have been proposed to compensate for the distortion but these compensation methods have been found to be impractical due to a combination of excessive time required for calculating the compensation and the uncertainty in the reading of the cardiac output due to the inaccuracy of the applied approximation methods. The use of indicator dilution techniques are disclosed, for example, in U.S. Pat. No. 4,572, 206 to Greddes et al.; U.S. Pat. No. 4,380,237 to Newbower; and U.S. Pat. No. 4,015,593 to Elings et al.
The thermal dilution technique relies on the use of a chilled thermal indicator that is introduced into the right atrium of the heart. The thermal indicator is then carried by the blood through the heart to the pulmonary artery. As the thermal indicator mixed with surrounding blood in the heart, it cools the temperature of the blood before the blood is pumped out of the heart and passes a thermistor. The thermistor measures the decrease in the blood temperature as the blood flows pasts the thermistor in the pulmonary artery. The time-temperature information derived from the thermistor is then used to calculate the cardiac output of the patient. The use of thermal dilution techniques are disclosed, for example, in U.S. Pat. No. 5,595,181 to Hubbard; U.S. Pat. No. 5,285,796 to Hughes; and U.S. Pat. No. 4,819,655 to Webler.
These invasive techniques present three primary disadvantages. First, the techniques require the insertion of catheter into the patients body. Normally, the catheter is inserted in the femoral artery and threaded into the heart. Since these invasive techniques for measuring cardiac output involve penetration of the skin by a catheter, they present an inherent undesirable risk of trauma and infection to the patient. Second, these techniques require the use of complex instrumentation which must be operated by skilled personnel. Finally, these techniques allow only intermittent measurement of the cardiac output since it is possible to obtain only one determination of cardiac output per injection.
The noninvasive techniques for measuring cardiac parameters include ballistocardiograpy, electrical impedance measurements, ultrasonics, phonocardiography, and vibrocardiography. The instrumentation typically involved in present noninvasive techniques for measuring cardiac output is complex, expensive, inconvenient to use and requires highly trained operators. The most common noninvasive technique is the measurement of the cardiovascular impedance change during a systolic downstroke. Such techniques are disclosed, for example, in U.S. Pat. No. 5,423,326 to Wang et al.; U.S. Pat. No. 4,898,176 to Petre; U.S. Pat. No. 4,947,852 to Nassi et al.; and U.S. Pat. No. 4,450,527 to Sramek. The models used in such electrical impedance techniques generally incorporate many simplifying assumptions which, depending upon the divergence of an actual living body form the assumed conditions of the model, may degrade the accuracy of these methods significantly.
Still other methods of cardiac output measurement are based upon the Fick principle. According to this principle, the rate of uptake or release of a substance to or from blood at the lung is equal to the blood flow past the lung and the content difference of the substance at each side of the lung. This can be expressed by the equation: EQU Uptake=Q(c.sub.2 -c.sub.1),
where Q is the blood flow (cardiac output), c.sub.2 is the content of the substance leaving from the lung and c.sub.1 is the content of the substance coming to the lung. Applying the relationship to carbon dioxide yields: EQU Q=VCO.sub.2 /(c.sub.v CO.sub.2 -c.sub.a CO.sub.2),
where VCO.sub.2 is the volume of carbon dioxide produced by the patient per unit time and c.sub.v CO.sub.2 and c.sub.a CO.sub.2 are the mixed venous and arterial carbon dioxide contents. Determination of VCO.sub.2 requires a volume measurement (e.g., via intergration of a flow signal or via a rotameter) and a fractional concentration measurment (e.g., via mass spectrometer or gas analyzer (infrared or polarographic)). This method for determining cardiac output unfortunately retains still presents the inherent disadvantages associated with the previously mentioned invasive methods as a pulmonary catheter is required to be inserted into the patient's heart to monitor the mixed venous oxygen saturation of the blood. An example of this method is disclosed in U.S. Pat. No. 4,949,724 to Mahutte, et al.
The Fick method has also been applied to carbon dioxide employing equation to obtain intermittent cardiac output. The c.sub.v CO.sub.2 is usually estimated from the partial pressure of carbon dioxide (P.sub.v CO.sub.2) which is typically obtained indirectly by rebreathing. Such methods are described in Davis, C.C., et al., Measurements of Cardiac Output in Seriously Ill Patients using a CO Rebreathing Method, Chest 73, 167, 1978; and Blanch, et al., Accuracy of an Indirect Carbon Dioxide Fick Method in Determination of the Cardiac Output in Critically Ill Mechanically Ventilated Patients, Int. Care Med 14, 131, 1988. A major disadvantage of this method is that it yields only intermittent cardiac output values since the partial pressure of carbon dioxide is estimated via rebreathing.
These prior art invasive and noninvasive techniques, detailed above, require great skill in application, carry inherent risks to the patient, and have degraded accuracy. Thus there still remains a need for a noninvasive technique that simply and easily determines cardiac output.